This invention relates generally to the continuous regeneration of an adsorbent such as activated carbon using microwave energy and more particularly concerns a method and apparatus in which sorbated adsorbent material is conveyed through a microwave cavity via a moving belt wherein the adsorbent is heated to strip the sorbates therefrom.
In industry, process streams carrying contaminants or other components are often purified by passing the stream in contact with an adsorbent. The contaminants or other components are adsorbed by the adsorbent, thereby removing them from the process stream. These adsorbed materials are referred to as adsorbates or simply sorbates. Thus, the term sorbated adsorbent refers to an adsorbent having adsorbed materials therein. In the course of cleansing process streams, the adsorbent will eventually become saturated with sorbates and be unable to adsorb further materials. Rather than simply being disposed of, a saturated adsorbent can be recycled through a process which desorbs or strips the sorbates from the adsorbent. Once the sorbates have been desorbed, the adsorbent is again capable of being used to cleanse process streams.
Such processes are generally referred to as regeneration because they renew or regenerate the adsorbing capacity of the treated adsorbent. In the case where the adsorbent is activated carbon, a distinction is made sometimes where low temperature processes (i.e., in the range of 200.degree.-400 .degree. F.) are referred to as regeneration and higher temperature processes (up to 1800.degree. F.) are referred to as reactivation. However, for the sake of clarity, the term "regeneration" as used herein, will include both low and high temperature desorbing processes. It is desirable to employ a regeneration process which is capable of stripping the sorbates on the plant site, thereby eliminating the need to ship the sorbated carbon off site for cleaning. Besides offering cost advantages, on site regeneration reduces the number of plant emissions which must be reported to the Environmental Protection Agency.
A typical method of regenerating a saturated adsorbent is to heat the adsorbent with a flow of hot gas such as steam or flue gases to a sufficiently high temperature at which the sorbate will be desorbed. The high temperature causes the sorbated matter to vaporize and pass from the adsorbent. The flow of the hot gas also purges the vaporized or desorbed materials from the system. This gas heating method has the problems of long regeneration times, low heating efficiency, requiring large amounts of purge gas, diluting the sorbate vapors with heating gases, and often generating resulting sorbate condensates containing a large fraction of water.
To avoid many of the problems associated with the hot gas heating method, microwave heating of the adsorbents such as activated carbon has been proposed. A simple approach to microwave heating is to place the carbon adsorbent into a bulk container and expose the container to microwave energy in order to heat the adsorbent to the regeneration temperature. However, this approach is still very inefficient and time consuming because it is a non-continuous or batch operation, wherein only relatively small amounts of adsorbent material can be regenerated during a cycle. This approach also presents difficulty in charging and discharging carbon in and out of the container, can experience agglomeration of carbon granules when treating carbon containing water, dirt and/or other solids, provides slow heating of the center of the carbon bed due to the limited penetration depth of the microwaves, and experiences heavy attrition of carbon due to excessive rough handling during loading and unloading.
One solution to the problem of batch operation is set forth in U.S. Pat. No. 4,737,610 to Kotsch et al. The Kotsch et al patent discloses a method and apparatus using a gravity-driven moving bed for the desorption of noxious materials from a carbonaceous adsorption agent. Saturated carbon or coke is fed into the regeneration unit via a dosing and closure unit 1. The coke falls into a quartz conduit 2 where it is heated by a microwave heating means. The coke then enters a desorption gas collector 6 where it builds up a free fill above a perforated conical plate 7. In the desorption gas collector 6, the coke is swept with an inert gas to apparently remove the desorbed noxious materials. In a second embodiment, the quartz conduit is replaced with a horizontal moving belt which conveys coke through a heating chamber prior to dumping the coke into the desorption gas collector 6 by gravity feed. While the Kotsch et al patent does not use a batch operation, it still faces the other operating problems mentioned above. Due to the free fall method of moving the carbon, there is a high degree of relative movement between the carbon granules and between the carbon granules and the walls of the container. This relative movement, tends to grind the carbon into smaller, less useful particles, thus producing attrition losses. The carbon is especially susceptible to attrition at the high temperatures involved with regeneration. The free falling carbon is also susceptible to agglomeration of granules when containing water, dirt and/or other solids. Furthermore, the universal desorption gas collector 6 is unable to selectivity recover the desorbed noxious materials.